Magnetooptical recording medium, method for producing the same, method for recording or reproducing information in or from the same

Abstract: A magnetooptical recording medium for information recording and information reproduction includes plural thin films, including at least a reproducing layer and a recording layer, laminated on a substrate. The reproducing layer is composed of a magnetic thin film which assumes a horizontally magnetized state at room temperature but assumes a vertically magnetized state above a predetermined temperature higher than the room temperature. The magnetization in the horizontally magnetized state is free from vertical orientation, including the magnetic wall. The recording layer is composed of a magnetic thin film which assumes a vertically magnetized state at room temperature and above the predetermined temperature.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a magnetooptical recording medium for effecting information recording and reproduction with laser light, a method for producing the recording medium, and a method for recording or reproducing information in or from the recording medium. Related Background Art For rewritable high-density information recording, keen interest is being paid to the magnetooptical recording medium in which information is recorded by forming a magnetic domain in a thin magnetic film with thermal energy of semiconductor laser and is reproduced by the magnetooptical effect, and, more recently, there is an increase in demand for increasing the recording density of such magnetooptical recording medium to attain a further increased recording capacity. The linear recording density of an optical disk, such as the magnetooptical recording medium, is principally determined by the S/N ratio of the reproducing layer, and is significantly dependent on the frequency of the signal bit train, the wavelength of the laser employed in the reproducing optical system, and the numerical aperture of the objective lens. More specifically, once the wavelength λ of the laser of the reproducing optical system and the numerical aperture NA of the objective lens are given, the bit frequency f of the detection limit is determined by the following equation: f = λ/2NA On the other hand, the track density is principally limited by the crosstalk, which is principally determined by the distribution (profile) of the laser beam on the medium surface and can also be represented by a function of (λ/2NA) as in the case of the bit frequency mentioned above. Consequently, for achieving a higher density in the conventional optical disk, it is necessary to shorten the wavelength of the laser in the reproducing optical system and to increase the numerical aperture NA of the objective lens. However, there are certain limits in such reduction of the wavelength of the laser and increase in the numerical aperture. EP-A-0 509 836 discloses an optical recording medium in which the recording layer has a magnetization perpendicular to the layer and the reproducing layer has a magnetization parallel to the layer in a temperature range up to a predetermined temperature and a magnetization perpendicular to the layer at temperatures greater than the predetermined temperature. During reproduction, only those parts of the medium which are heated by the reproducing light spot above the predetermined temperature will produce a measurable magneto-optic signal. SUMMARY OF THE INVENTION In consideration of the foregoing, the object of the present invention is to provide a magnetooptical recording medium capable of increasing the recording density irrespective of the wavelength of the laser or the numerical aperture of the objective lens, a method for producing such recording medium, and an information record/reproducing method for such recording medium. According to a first aspect of the present invention, there is provided a magnetooptical recording medium comprising a plurality of laminated layers comprising a reproducing layer having a magnetization parallel to the layer at temperatures up to a predetermined temperature and a magnetization perpendicular to the layer at temperatures at or greater than said predetermined temperature; and a recording layer having a magnetization perpendicular to the layer at temperatures up to, at and greater than said predetermined temperature; the medium being characterized in that said reproducing layer is magnetically oriented to have an unidirectional magnetization within the plane of the layer when the magnetization of the reproducing layer is parallel to the layer. According to a second aspect of the present invention, there is provided a method for manufacturing a magnetooptical recording medium comprising a plurality of laminated layers comprising a reproducing layer having a magnetization parallel to the plane of the layer at temperatures up to a predetermined temperature and a magnetization perpendicular to the layer at temperatures at or greater than said predetermined temperature; and a recording layer having a magnetization perpendicular to the layer at temperatures up to, at and greater than said predetermined temperature; wherein said method comprises the steps of forming a reproducing layer; and forming a recording layer; characterized in that during said reproducing layer forming step, a magnetic field is applied in a direction parallel to the plane of the reproducing layer to align the magnetization of the reproducing layer in a predetermined direction within the plane of the reproducing layer when the magnetization of the reproducing layer is parallel to the layer. According to a third aspect of the present invention, there is provided a method for recording information on a magnetooptical recording medium comprising a plurality of laminated layers comprising a reproducing layer having a magnetization parallel to the layer at temperatures up to a predetermined temperature and a magnetization perpendicular to the layer at temperatures at or greater than said predetermined temperature; and a recording layer having a magnetization perpendicular to the layer at temperatures up to, at and greater than said predetermined temperature; said method comprising the steps of irradiating said medium with laser light to heat a portion of the medium to a temperature less than said predetermined temperature; and recording information on the irradiated portion of the medium; characterized in that said method includes a step of applying to the irradiated portion of the medium an external magnetic field in a direction parallel to the plane of the reproducing layer to align the magnetization of the reproducing layer in a predetermined direction within the plane of the reproducing layer when the magnetization of the reproducing layer is parallel to the layer. According to a fourth aspect of the present invention, there is provided a method for reproducing information from a magnetooptical recording medium comprising a plurality of laminated layers comprising a reproducing layer having a magnetization parallel to the layer at temperatures up to a predetermined temperature and a magnetization perpendicular to the layer at temperatures at or greater than said predetermined temperature; and a recording layer having a magnetization perpendicular to the layer at temperatures up to, at and greater than said predetermined temperature; the direction of magnetization of the recording layer being dependent on information stored in the recording layer, said method including the steps of irradiating a portion of said reproducing layer so as to heat said reproducing layer to a temperature at or greater than said predetermined temperature, so that said portion of the reproducing layer has a magnetization perpendicular to the layer coupleable to the magnetization in the corresponding region of said recording layer; and detecting reflected light from the reproducing layer to read out the information in said recording layer transferred to the irradiated portion of the reproducing layer, characterized in that said method further includes the step of applying a magnetic field in a direction parallel to the reproducing layer to the irradiated portion of said reproducing layer to align the magnetization of the reproducing layer in a predetermined direction within the plane of the reproducing layer when the magnetization of the reproducing layer is parallel to the layer. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are schematic cross-sectional views of magnetooptical recording medium of the present invention, wherein Fig. 1A is a view showing the magnetized state of a recording layer and a reproducing layer while Fig. 1B is a view showing configuration of layers; Fig. 2 is a schematic view showing magnetic domains in the reproducing layer of a conventional magnetooptical recording medium; Figs. 3A and 3B are schematic views showing the state of magnetic wall when magnetic domains are formed in the reproducing layer in a conventional magnetooptical recording medium; Figs. 4A and 4B are schematic views showing the state of magnetization when magnetic domains are formed in the recording layer by the magnetic domains formed in the reproducing layer in a conventional magnetooptical recording medium; Figs. 5A and 5B are schematic views showing the state of magnetization when magnetic domains are formed in the recording layer by a one-directional magnetic domain of the reproducing layer in the magnetooptical recording medium of the present invention; Fig. 6 is a chart showing the temperature-dependent change of demagnetizing energy 2πMs 2 and vertical magnetic anisotropic energy Ku of the reproducing layer; and Figs. 7A and 7B are schematic views showing the magnetooptical reproducing method, wherein Fig. 7A shows a case employing the magnetooptical recording medium of the present invention, while Fig. 7B shows a case employing a conventional magnetooptical recording medium in which magnetic domains are formed in the reproducing layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The magnetooptical recording medium of the present invention is provided, on a substrate, at least with a first magnetic layer (reproducing layer) which assumes the state of a horizontally (in-surface) magnetized film at room temperature but assumes the state of a vertically magnetized film above a predetermined temperature higher than the room temperature, and a second magnetic layer (recording layer) which assumes the state of a vertically magnetized film at room temperature and above said predetermined temperature, with a Curie temperature lower than that of the first magnetic layer. In the following the present invention will be clarified in detail, with reference to the attached drawings, in which Figs. 1A and 1B are schematic views showing the configuration of layers of the magnetooptical recording medium of the present invention, Fig. 2 is a schematic view showing the state of magnetic domains in case that magnetic domains are formed in the reproducing layer in a conventional magnetooptical recording medium, Figs. 3A and 3B are schematic views showing the state of magnetic wall in case that magnetic domains are formed in the reproducing layer in a conventional magnetooptical recording medium, Figs. 4A and 4B are schematic views showing the state of magnetization in case that magnetic domains are formed in the recording layer by the magnetic domains formed in the reproducing layer in a conventional magnetooptical recording medium, Figs. 5A and 5B are schematic views showing the state of magnetization in case that magnetic domains are formed in the recording layer in the presence of a one-directional magnetic domain in the reproducing layer, Fig. 6 is a chart showing the temperature-dependent change of demagnetizing energy 2πMs 2 and vertical magnetic anisotropic energy Ku of the reproducing layer, and Figs. 7A and 7B are schematic views showing the magnetooptical reproducing method. (1) Structure and composition of medium The reproducing layer of the magnetooptical recording medium of the present invention is preferably composed of a material having a compensation temperature between the room temperature and the Curie temperature, for example an amorphous rare earth-iron group alloy, such as GdCo, GdFeCo, GdTbFeCo, GdDyFeCo or NdGdFeCo. Magnetization of the reproducing layer in the horizontally magnetized state is free from vertical orientation, including the magnetic wall. The recording layer is preferably composed of a material having large vertical magnetic anisotropy and capable of stably maintaining the magnetized state, for example an amorphous rare earth-iron group alloy such as TbFeCo, DyFeCo or TbDyFeCo; a garnet; a periodically structured platinum group-iron group film such as Pt/Co or Pd/Co; or a platinum group-iron group alloy such as PtCo or PdCo. These magnetic layers may additionally contain an element such as Cr, Ti or Pt for improving corrosion resistance. Also, in addition to these magnetic layers, there may be provided a dielectric layer for example of SiN, AlN x , AlO x , TaO x or SiO x in order to increase the interference effect. Furthermore, for improving the thermal conductivity, there may be added a layer for example of Al, AlTa, AlTi or Cu. Furthermore, there may be additionally provided an intermediate layer for regulating the exchange coupling force or the statomagnetic coupling force, or an auxiliary layer for assisting the recording or reproduction. Furthermore there may be added a protective coated layer consisting of a dielectric material as mentioned above or of polymer resin. In the following there will be explained the recording and reproducing methods of the present invention. (2) Data recording At first data signal is recorded in the recording layer of the magnetooptical recording medium of the present invention, as shown in Fig. 1A. The recording is achieved either by modulation of an external magnetic field under the irradiation of laser light of a power capable of heating the recording layer at least to the Curie temperature, or, after information erasure, by modulation of laser power under the application of a magnetic field in the recording direction. In this operation, if the intensity of the laser light is so determined, in consideration of the linear velocity of the recording medium, that only a predetermined area of the recording layer within the light spot reaches the Curie temperature or the vicinity thereof, there can be formed a recorded magnetic domain smaller than the diameter of the light spot, so that the signal can be recorded with a frequency below the diffraction limit of the light. The reproducing layer in horizontally magnetized state may form recycling magnetic domains as shown in Figs. 2, 3A and 3B, in order to reduce the demagnetizing energy. The boundary of such magnetic domains may be in a state where the magnetic moment rotates horizontally (Néel magnetic wall), or a state where the magnetic moment rotates vertically (Bloch magnetic wall) as shown in Figs. 3A and 3B. In the recording operation, if a portion to be recorded of the recording layer is adjacent to the magnetic wall of the reproducing layer as shown in Figs. 4A and 4B, the vertically oriented magnetization of the reproducing layer influences the magnetization generating process in the recording layer, thereby facilitating the formation of inverted magnetization within the magnetic domain of the recording layer. Formation of such uneven magnetic domain (such as a microdomain) increases the noise in the reproducing process, thereby hindering accurate readout of the recorded information. In contrast, in the present invention, the reproducing layer is one-directionally magnetized as shown in Figs. 5A and 5B, or, even in the presence of a magnetic wall, the magnetization therein is maintained horizontally. Consequently the recording layer is maintained free from undesirable influence from the magnetic wall, and does not generate microdomains. For this reason the magnetic domains can be formed in proper manner. (3) Data reproduction At the data reproduction, the recording medium is irradiated with reproducing laser light from the side of the reproducing layer, whereby the irradiated portion becomes heated. Since the recording medium is moved with a constant velocity, there is generated a temperature distribution in the moving direction of the medium, whereby a part within the light spot reaches a high temperature. In a single-layered magnetic film, it is already known that the main direction of magnetization is determined by an effective vertical magnetic anisotropy constant K⊥ defined by: K⊥ = Ku - 2πMs 2 wherein Ms is the saturation magnetization and Ku is the vertical magnetic anisotropy constant. A vertically magnetized film or a horizontally magnetized film is obtained respectively when K⊥ is positive or negative. 2πMs 2 indicates the demagnetizing energy. The reproducing layer of the present invention is a horizontally magnetized film at the room temperature (RT) because, as shown in Fig. 6; Ku < 2πMs 2 , K⊥ < 0. However, at the reproducing operation, it becomes a vertically magnetized film because the heating reduces Ms of the reproducing layer, thus drastically reducing 2πMs 2 and inverting the relationship with Ku as: Ku > 2πMs 2 , K┴ > 0. When such thin magnetic film is laminated directly on a horizontally magnetized film or via an intermediate layer, the apparent Ku varies due to the influence of the exchange coupling force or the statomagnetic coupling force from these layers, but there can still be attained the horizontally magnetized state at the room temperature and the vertically magnetized state at the elevated temperature even when laminated with other layers, by suitably selecting the temperature range of vertical magnetization in the single-layered film higher or lower according to the above-mentioned influence. Thus, by laminating the recording layer consisting of the vertically magnetized film with the reproducing layer of which direction of magnetization varies from horizontal direction to vertical direction, only a high temperature portion of the light spot assumes the state of vertical magnetization to cause exchange coupling between the recording and reproducing layers, whereby the magnetization of the recording layer is transferred. It is therefore rendered possible, by detecting the reflected light of the irradiating laser light, to reproduce the signal with a frequency below the diffraction limit of the light. Also in the present invention, as the reproduction can be achieved without the influence of bits adjacent in the track or radial direction, the signals recorded with an increased linear recording density and an increased track density can be reproduced with a satisfactory C/N ratio. In the foregoing explanation, the layers are assumed to be magnetically coupled by exchange coupling, but they may also be magnetically coupled by statomagnetic coupling. Furthermore where may be provided an intermediate layer of a different Curie temperature, for achieving sharper signal transfer. In the above-explained data reproducing process, a Bloch magnetic wall eventually present in the reproducing layer within the light spot as shown in Fig. 7B may generate a vertically magnetized component in the mask area, thus causing a noise in the data reproduction. In the present invention, however, such noise generation does not occur because the reproducing layer is one-directionally magnetized, without magnetic wall, at least within the laser spot as shown in Fig. 7A. The above-mentioned one-directionally magnetized state, as explained in (2) data recording and (3) data reproduction, can be attained by a uniaxial anisotropy obtained by the application of a horizontal magnetic field at the film formation, or by the application of a horizontal magnetic field to the portion irradiated with the laser light at the recording or reproducing operation. The magnitude of such magnetic field to be horizontally applied should be so large as to one-directionally orient the horizontal magnetization but so small as not to detrimentally influence the magnetic domain formation at the recording operation, by the inclination of the magnetizations of the recording and reproducing layers toward the horizontal direction. Also the state free from a magnetic wall having vertical magnetization in the reproducing layer, or the state of recording or reproduction in which the magnetization of the reproducing layer is oriented in a horizontal direction, only needs to be realized at least within the laser light spot. In the following the present invention will be described by examples thereof. [Example 1] Targets of Si, Tb, Gd, Fe and Co were mounted in a DC magnetron sputtering apparatus, then a polycarbonate substrate was fixed on a substrate holder, and the chamber was evacuated with a cliopump to high vacuum of 1 x 10 -6 Pa or less. After the introduction of Ar gas to 0.4 Pa, there were formed, in succession, a SiN interference layer of a thickness of 780 Å (1Å = 10 -1 nm), a GdFeCo reproducing layer, a TbFeCo recording layer and a SiN protective layer of a thickness of 700 Å to obtain a configuration shown in Fig. 1B. The SiN layer was formed with DC reactive sputtering under the introduction of N 2 gas in addition to Ar gas. The GdFeCo layer and TbFeCo layer were formed by applying DC power to the Gd, Fe, Co and Tb targets. The GdFeCo reproducing layer had a thickness of 400 Å and such a composition as to have the compensation temperature at 280°C and the Curie temperature at 350°C or higher. The TbFeCo recording layer had a thickness of 400 Å and such a composition as to be TM rich at the room temperature and to have the compensation temperature below the room temperature and the Curie temperature at 220°C. Thus prepared magnetooptical recording medium was subjected to the following measurement of record/reproducing characteristics. The measuring apparatus employed an objective lens with N.A. = 0.55, and a laser wavelength of 780 nm. The recording operation was conducted with a power of 8 - 10 mW, and a linear velocity of 9 m/s (revolution 2400 rpm, radius 36 mm). A carrier signal of 5.8 - 15 MHz was recorded in the recording layer by the magnetic field modulation method with a recording magnetic field of ±150 Oe, under the application of a horizontal auxiliary magnetic field of 20 Oe, and the dependence of C/N ratio on the recording frequency was investigated. The reproducing power was so adjusted as to maximize the C/N ratio. The obtained results are shown in Table 1. [Example 2] A magnetooptical recording medium was prepared on a polycarbonate substrate by forming a GdFeCo reproducing layer and a TbFeCo recording layer in a similar manner as in the example 1. However, the TbFeCo recording layer had a thickness of 300 Å and such a composition as to be TM rich at the room temperature and to have the compensation temperature below the room temperature and the Curie temperature at 240°C. This magnetooptical recording medium was subjected to the recording of a carrier signal of 8 - 15 MHz under the application of a horizontal magnetic field and the signal reproduction, in a similar manner as in the example 1, except that the recording magnetic field was 160 Oe and the horizontal auxiliary magnetic field was 50 Oe. The reproducing power was so adjusted as to maximize the C/N ratio. The obtained results are shown in Table 1. [Example 3] On a polycarbonate substrate, there were formed in succession, a SiN interference layer of a thickness of 780 Å, a GdFeCo reproducing layer, a DyFeCo recording layer and a SiN protective layer of a thickness of 700 Å in a similar manner as in the example 1. At the formation of the reproducing layer, an external magnetic field of 500 Oe was applied horizontally, in order to generate a uniaxial magnetic anisotropy in the horizontal direction. The GdFeCo reproducing layer had a thickness of 400 Å and such a composition as to have the compensation temperature at 270°C and the Curie temperature at 350°C or higher. The DyFeCo recording layer had a thickness of 300 Å and such a composition as to be TM rich at the room temperature and to have the compensation temperature below the room temperature and the Curie temperature at 200°C. The recording magnetic field was ±150 Oe, and the reproducing power was so adjusted as to maximize the C/N ratio. The obtained results are shown in Table 1. [Example 4] On a polycarbonate substrate, there were formed, in succession, a SiN interference layer of a thickness of 780 Å, a GdFeCo reproducing layer, a TbFeCo recording layer, and a SiN protective layer of a thickness of 700 Å in a similar manner as in the example 1. The GdFeCo reproducing layer had a thickness of 400 Å and such a composition as to have the compensation temperature at 260°C and the Curie temperature at 350°C or higher. The TbFeCo recording layer had a thickness of 400 Å and such a composition as to be TM rich at the room temperature and to have the compensation temperature below the room temperature and the Curie temperature at 200°C. The recording magnetic field was ±150 Oe. The reproduction was conducted under the application of a weak horizontal magnetic field of about 15 Oe, and the reproducing power was so adjusted as to maximize the C/N ratio. The obtained results are shown in Table 1. [Reference Example 1] The example 1 was reproduced in the preparation of the magnetooptical recording medium and in the measurement of record/reproducing characteristics, however, without the application of the auxiliary horizontal magnetic field in the film formation, recording or reproducing operation. The recording magnetic field was ±150 Oe, and the reproducing power was so adjusted as to maximize the C/N ratio. The obtained results are shown in Table 1.

Claims: A magneto-optical recording medium comprising a plurality of laminated layers comprising: a reproducing layer (1) having a magnetization parallel to the layer at temperatures up to a predetermined temperature and a magnetization perpendicular to the layer at temperatures at or greater than said predetermined temperature; and a recording layer (2) having a magnetization perpendicular to the layer at temperatures up to, at and greater than said predetermined temperature; the medium being characterized in that: said reproducing layer (1) is magnetically oriented to have an unidirectional magnetization within the plane of the layer when the magnetization of the reproducing layer is parallel to the layer. A magneto-optical recording medium according to claim 1, wherein the reproducing layer (1) has a compensation temperature between room temperature and the Curie temperature of the reproducing layer (1). A magneto-optical recording medium according to either of the preceding claims, including a coupling adjustment layer between the reproducing layer (1) and the recording layer (2). A magneto-optical recording medium wherein the reproducing layer (1) and recording layer (2) comprise rare earth-transition metal alloys. A method for manufacturing a magneto-optical recording medium comprising a plurality of laminated layers comprising: a reproducing layer (1) having a magnetization parallel to the plane of the layer at temperatures up to a predetermined temperature and a magnetization perpendicular to the layer at temperatures at or greater than said predetermined temperature; and a recording layer (2) having a magnetization perpendicular to the layer at temperatures up to, at and greater than said predetermined temperature; wherein said method comprises the steps of: forming a reproducing layer (1); and forming a recording layer (2); characterized in that during said reproducing layer forming step, a magnetic field (Hb) is applied in a direction parallel to the plane of the reproducing layer (1) to align the magnetization of the reproducing layer (1) in a predetermined direction within the plane of the reproducing layer (1) when the magnetization of the reproducing layer is parallel to the layer. A method for recording information on a magneto-optical recording medium comprising a plurality of laminated layers comprising: a reproducing layer (1) having a magnetization parallel to the layer at temperatures up to a predetermined temperature and a magnetization perpendicular to the layer at temperatures at or greater than said predetermined temperature; and a recording layer (2) having a magnetization perpendicular to the layer at temperatures up to, at and greater than said predetermined temperature; said method comprising the steps of: irradiating said medium with laser light to heat a portion of the medium to a temperature less than said predetermined temperature; and recording information on the irradiated portion of the medium; characterized in that said method includes a step of applying to the irradiated portion of the medium an external magnetic field (Hb) in a direction parallel to the plane of the reproducing layer (1) to align the magnetization of the reproducing layer (1) in a predetermined direction within the plane of the reproducing layer (1) when the magnetization of the reproducing layer is parallel to the layer. A method according to claim 6, wherein said recording step comprises applying an external magnetic field (Hb) in a direction perpendicular to the layers (1, 2) modulated in accordance with information to be recorded on the irradiated portion of the medium, the magnitude of said perpendicular field (Hb) being greater than that of the in-plane field. A method for reproducing information from a magneto-optical recording medium comprising a plurality of laminated layers comprising: a reproducing layer (1) having a magnetization parallel to the layer at temperatures up to a predetermined temperature and a magnetization perpendicular to the layer at temperatures at or greater than said predetermined temperature; and a recording layer (2) having a magnetization perpendicular to the layer at temperatures up to, at and greater than said predetermined temperature; the direction of magnetization of the recording layer (2) being dependent on information stored in the recording layer (2), said method including the steps of: irradiating a portion of said reproducing layer (1) so as to heat said reproducing layer (1) to a temperature at or greater than said predetermined temperature, so that said portion of the reproducing layer (1) has a magnetization perpendicular to the layer coupleable to the magnetization in the corresponding region of said recording layer (2); and detecting reflected light from the reproducing layer (1) to read out the information in said recording layer transferred to the irradiated portion of the reproducing layer (1), characterized in that said method further includes the step of applying a magnetic field (Hb) in a direction parallel to the reproducing layer (1) to the irradiated portion of said reproducing layer (1) to align the magnetization of the reproducing layer (1) in a predetermined direction within the plane of the reproducing layer (1) when the magnetization of the reproducing layer is parallel to the layer. A method according to any one of claims 5 to 8, wherein the reproducing layer (1) has a compensation temperature between room temperature and the Curie temperature of the reproducing layer (1). A method according to any one of claims 5 to 9, wherein the reproducing layer (1) and recording layer (2) are composed of rare earth-transition metal alloys. A method according to any one of claims 5 to 10, including a coupling adjustment layer between the reproducing layer (1) and the recording layer (2).

Inventors: FUJII EIICHI, NISHIMURA NAOKI, FUJII, EIICHI, NISHIMURA, NAOKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: G11B11
IPC codes: G11B11